siberian iris (198873 bytes)

The Timing of Dividing Perennials


By Carl Wilson, Extension Horticulturist

Gardeners wanting a low maintenance garden should not choose perennials that may require division every couple years. Examples are:

Achillea sp., (Yarrow)

Aster x frikartii, (Frikart's aster or Wonder of Staffa)

Centaurea montana, (Mountain bluet)

Centranthus ruber, (Juupiter's beard or Red valerian)

Coreopsis verticillata, (Threadleaf coreopsis)

Dianthus deltoides, (Maiden pink)

Geum hybrids, (Avens)

Heuchera micrantha ‘Palace Purple’, (Coral bells)

Iris bearded, (Bearded or German iris)

Lychnis coronaria, (Rose campion)

Veronica spicata. (Spike speedwell)

Many perennial flowers may need division only every 4-5 or even 6-10 years.

Some, such as Dictamnus albus, (gasplant), Gypsophila paniculata, (baby's breath), Paeonia (peony) hybrids and Perovskia atriplicifolia, (Russian sage), may require division only every 10 years or more. Note that all these times depend on growing conditions, particularly the amount of irrigation and fertilizer applied.

The common wisdom is that spring-flowering perennials should be divided in the autumn, and autumn-flowering perennials in the spring. Spring flowering plants divided right after flowering in April and May have more time for establishment before winter, a factor in our climate. Very early spring flowering plants may even be divided before flowering if the gardener doesn’t mind sacrificing a bloom year. Autumn flowering plants, especially the ornamental grasses, should be divided in the spring.

  • Summer is generally a poor time to divide perennials with the exception of Iris that respond well to July/August divisions.

  • Fleshy rooted perennials such as Paeonia (peony), Papaver orientale (Oriental poppy), and Iris siberica (Siberian iris) are best divided in autumn.

  • Woody or taprooted perennials such as Aruncus (goatsbeard), Asclepias (butterfly weed),
    Ceratostigma (plumbago), Cimicifuga (bugbane) and Echinops (globe thistle) should be divided in early spring before top growth emerges.

Photograph of Siberian Iris courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010